A microbial fuel cell was tested in marine sediment samples collected from known methane hydrate sites to determine whether power levels of 0.1 - 1 W, typically supplied by batteries in seafloor instrumentation, can be achieved. This fuel cell oxidizes biologically produced sulfide in the sediment and reduces dissolved oxygen in the water column to produce electricity. The specific objectives of this study were to demonstrate feasibility of concept; identify the most probable oxidation reactions that will occur on an anode exposed to the microbial metabolites and identify the bacteria that produce these reactants; quantify important system parameters including exchange current density and charge transfer coefficient; and establish baseline fuel cell power output, potential, and current density. Sediment samples used in this study were taken from sites on Blake Ridge, Cascadia Margin, and the Gulf of Mexico. DNA extracted from Gulf of Mexico sediments closely matched the sulfate-reducing bacteria Desulfotomaculum. Cyclic voltammetry and sampled-current voltammetry techniques were applied to sulfide solutions produced by this bacterium in Bactosulfate API enrichment media. The most probable anode reaction was determined to be the oxidation of hydrogen sulfide to elemental sulfur. At higher potentials, iron sulfide may also be oxidized. Linear potential scans of a graphite electrode immersed in oxygenated synthetic seawater suggest that oxygen reduction to water dominates the cathode reactions. Fuel cells operated in sediment samples were able to generate up to 0.010 W/m2 of power during short discharges. This power density is similar to data reported for microbial fuel cells tested in situ in estuarine environments (Reimers et al., 2001). Higher currents were observed in a laboratory setup where fuel cell electrodes were immersed in separate compartments, filled with a liquid culture of sulfate-reducing bacteria and with synthetic seawater, that were separated by a tube of sediment. Power generated by this cell was 0.018 W/m2 .The mass transfer of sulfide to the sediment electrode was found to be the current limiting process. Tafel plots of the fuel cell current-voltage data were employed to estimate the values of the exchange current density and charge transfer coefficient. The charge transfer coefficient was 0.98 and the average value of the exchange current density was 5.75 mA/m2.

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Theses for the degree of Master of Science (University of Hawaii at Manoa). Biosystems Engineering ; no. 3919

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